Estrogens are a group of hormones that are involved in many critical physiological functions in the human body. Estrogens function to facilitate the development of the female sexual organs, preparing the breast and uterus for pregnancy and breast feeding after childbirth. Estrogens also play important roles in maintaining proper cardiovascular function and bone density. It is well-known that Estrogens can stimulate cell proliferation and hence may increase a risk of developing cancers for females, especially breast cancer and uterus cancer.
Estrogens regulate cell functions by binding to estrogen receptors in target cells. Two types of estrogen receptors (ERs), i.e., ER-α and ER-β, were found in human cells. ER-α and ER-β have similar protein structures, each of which possesses three independent but interacting functional domains: the N-terminal domain (A/B domain), the central DNA-binding domain (C domain), and the C-terminal ligand-binding domain (D/E/F domain). The N-terminal domain has a ligand-independent activation function (AF-1), which can interact with coactivators and transcriptionally activate the target genes in the absence of ligands. The DNA binding-domain plays important roles in the dimerization of receptors and binding to specific DNA sequence. The C-terminal ligand-binding domain may mediate ligand binding and has a ligand-dependent transcriptional activation function (AF-2), which may activate gene transcription in the presence of ligands.
The full-length ER-α was identified as a 66 kDa protein and referred to as ER-α66. ER-α66 contains all three function domains. A splice variant of ER-α66 was later discovered and named ER-α46. ER-α46 has a molecular weight of about 46 kDa and lacks the N-terminal AF-1 domain of ER-α66. Recently, a novel 36 kDa ER-α variant, ER-α36, was identified, which lacks the N-terminal AF-1 domain and the C-terminal AF-2 domain of ER-α66 (Wang et al., Biochem. Biophys. Res. Commun. 336, 1023-1027 (2005)).
ER-α66 is believed to mediate estrogen-stimulated cell proliferation via transcriptional activation of the target genes. Binding of estrogen to ER-α66 activates the transcriptional activation domain of ER-α66 to stimulate the expression of downstream target genes and eventually leads to cell proliferation. ER-α46 was proved to mediate membrane-initiated and estrogen-stimulated rapid NO synthesis (Li et al., Proc. Natl. Acad. Sci. USA 100:4807-4812 (2003)). It was also shown that ER-α46 lacking the AF-1 domain, inhibits the AF-1 activity of ER-α66 (Flouriot, G., EMBO, 19, 4688-4700, (2000)). Since ER-α36 lacks both the AF-1 and AF-2 transcriptional activation domains, it may act as a dominant-negative inhibitor to inhibit both AF-1 and AF-2 functions of ER-α and ER-β. In addition, ER-α36 is localized primarily on the cell membrane and mediates the membrane-initiated mitogenic estrogen signaling that stimulates cell proliferation. (Wang et al., Biochem. Biophys. Res. Commun. 336, 1023-1027 (2005)); Wang et al., Proc. Natl. Acad. Sci. U.S.A. 103: 9063-9068 (2006)).
Extensive studies have shown that estrogen signaling is mediated via the classic nucleus transcriptional activation pathways as well as the non-classic membrane-initiated signaling pathways. It seems that ER-α66 and ER-α46 function primarily in the nucleus while ER-α36 functions mainly outside of the nucleus.
It was also shown that ER-α36 lacks Helix 8-12 of the ligand-binding domain contained in the original ER-α66, which totally changes the ligand binding specificity of ER-α36. Thus, ER-α36 may bind to ligands different from those to which ER-α66 and ER-β bind.
As estrogen receptor related diseases continue to affect many individuals, there remains an urgent need to develop novel compounds and pharmaceutical compositions thereof useful for preventing and/or treating such diseases.